When supply is greater than demand, prices drop; when demand is greater than supply, prices rise. Demand: Q=100-10P Supply: Q=10P (a) Solve for the equilibrium price and quantity.(b) Fill out the following table. The law of demand posits that demand declines when prices rise for a given resource, product, or commodity. Demand increases as prices fall. A change in price results in the same change in demand. Price elasticity for any two price points can be calculated using the following formula. For a demand curve, quantity falls when price increases. So the change in demand is negative if the price change is positive, and vice versa. The price of jellybeans goes up.